There is a conventional infrared array sensor researched and developed in various places, which are formed by using micromachining technologies and includes a base substrate and an array of pixel parts, having an infrared absorption part, located at one surface side of the base substrate (see Japanese Patent Application Publication No. P2001-309122A (hereinafter referred to as a “patent document”)).
An infrared image sensor disclosed in the patent document includes a silicon substrate, and pixel forming regions are formed on a surface of the substrate. Sensor elements (e.g., bolometer sensor elements) are also placed on the pixel forming regions, respectively. The patent document also describes that each bolometer sensor element can be replaced with a pyroelectric sensor element or a thermopile sensor element.
Specifically, each of the pixel forming regions is divided into four regions (hereinafter referred to as “segmented regions”), and four sensor elements are placed in the four segmented regions, respectively. Each of the four sensor elements is a membrane structure arranged so as to cover an opening of a depressed portion formed on a surface of the silicon substrate. The structure includes a SiO2 thin film, metallic thin film resistors (resistive bolometers) located on the thin film, a SiO2 thin film (second SiO2 thin film) located on the resistors, and an absorbing film located on the second SiO2 thin film.
Thus, both ends of each sensor element formed in the four segmented regions are extended in a direction along a diagonal line of its own segmented region. The four sensor elements are connected in series by three conductive patterns on a surface of the silicon substrate (end face between depressed portions). Thereby, one pixel output is given by a sum of outputs of four sensor elements, and accordingly each pixel output can be increased. In comparison with a sensor of which each pixel forming region is provided with one sensor element, it is possible to reduce each heat capacity of the four segmented regions and a time constant (a thermal time constant). Therefore, the response speed can be increased.
However, in the infrared image sensor, each of the pixel forming regions is divided into four segmented regions by a cross-shaped slit, and accordingly an area of a membrane structure per pixel is decreased by the slit. It is therefore difficult to increase the sensitivity of the sensor.
If each thickness dimension of the membrane structures is increased, the sensitivity can be increased, but the response speed is reduced because each heat capacity of the membrane structures is increased. Conversely, if each thickness dimension of the membrane structures is decreased, the response speed is increased, but each membrane structure may warp in addition to an increase of the sensitivity, which thereby causes a decrease in yield by breakage in production. The decrease of structure stability may also cause reduction in sensitivity.
Each of the sensor elements formed in the four segmented regions is supported by the silicon substrate through two linear arms extended in a direction along a diagonal line of its own segmented region. Accordingly, each membrane structure may be deformed by external force such as vibration or the like.